Digital inkjet printing is becoming an increasingly popular method for the production of fine graphic images for advertising, due to its low implementation cost and versatility in comparison with traditional techniques such as lithographic and screen printing. Inkjet printers comprise one or more printheads that include a series of nozzles through which ink is ejected onto a substrate. The printheads are typically provided on a printer carriage that traverses the print width (moves back and forth across the substrate) during the printing process.
Two main ink chemistries are used: inks that dry by solvent evaporation and inks that dry by exposure to actinic radiation (typically UV radiation). Wide-format solvent-based inkjet printers are an economic route into the industry as they are a relatively low-cost option compared to the more complex machines employed for UV curing. Solvent-based inkjet printing also has other advantages. As well as the lower cost, the ink films produced are thinner (and therefore flexible) and yield a good quality natural looking image with a gloss finish. Furthermore, it is difficult to achieve very high pigment loadings in UV curable inks due to the high viscosity of the ink: if too much pigment is added, the ink becomes too viscous and cannot be jetted. In contrast, solvent-based inks include a high proportion of solvent and therefore have a lower viscosity, which means that higher pigment loadings can be tolerated. In addition, the printed film produced from solvent-based inkjet inks is formed predominantly of pigment along with comparatively few other solids that are included in the ink. The pigment is therefore largely unobscured, resulting in intense, vivid and vibrant colours and a large colour gamut.
However, there are some limitations to solvent-based inkjet technology. In particular, solvent-based inks may not adhere to certain types of substrate, particularly non-porous substrates such as plastics, and the cured films have poor resistance to solvents. However, the printing of high-quality low-intercolour-bleed inkjet images with good mechanical and chemical resistance properties onto less receptive substrates is a requirement in many industrial printing applications. Such substrates include rigid PVCs, polyester and polycarbonate.
In addition, inkjet inks capable of being printed at small drop sizes and hence producing the required high image quality have a number of formulation constraints, including the requirement for low viscosity in order to be printed through these low drop volume printheads. This is easily achievable with solvent-based ink compositions due the inherent low viscosity of the organic solvents used. However, these types of ink often have poor chemical and scratch resistance and can have difficulty in drying on these less receptive materials.
To give adequate head stability, solvent-based inkjet inks are typically formulated with relatively low evaporation rate solvents and the inks rely on both evaporation and imbibition into the substrate to give adequate pinning of the ink droplets to fix the image quality (the term “pinning” is used in the art to mean arresting the flow of the ink by treating the ink droplets quickly after they have impacted onto the substrate surface). If the solvent is not able to penetrate into the substrate after deposition of the ink droplet, the rate of viscosity increase is too slow resulting in excessive bleed. If faster evaporating solvents are used in an attempt to overcome this problem head stability can be compromised through solvent loss leading to build up of dried ink deposits on the head face plate. In addition the use of faster solvent blends can also give rise to undesirable Marangoni effects, where faster evaporation at the edge of the ink deposit gives rise to a surface tension gradient which drives a bulk flow to the print edges (the so-called “coffee stain effect”).
Conventional UV-curable inkjet inks have excellent head stability and typically have better mechanical and chemical resistance properties than solvent-based inks. Image quality is less affected by the nature of the substrate as the droplet is cured or partially pinned by exposure to ultraviolet light immediately after deposition. However, the inherently higher viscosity of the radiation-curable materials greatly restricts the formulation latitude and in practice inks with suitably low viscosities have poor mechanical and chemical resistance properties.
Hybrid radiation-curable/solvent-containing inkjet inks (see, for example, international patent application no. PCT/GB2010/051384) can overcome most of the above limitations and allow UV-curable inks to be formulated to meet the low viscosity requirements (previously met by purely solvent-based inks) whilst still maintaining the chemical resistance and mechanical properties (as previously provided mainly by UV-curable inks) required for these industrial applications. However, in common with purely solvent-based inks, there are limitations to the substrate types that can be used. This is because, like solvent-based inks, hybrid inks also fix image quality by solvent loss and imbibition; this means that image quality is reduced on non solvent receptive substrates and so presents a problem. Accordingly, there remains a need in the art for approaches which address these problems.